The present invention relates to optical fiber attenuators for attenuation of optical signal communications and for the reduction of modal interference in such optical fiber attenuators.
It is common knowledge in the art that an optical fiber doped with transition metal elements will exhibit a high transmission loss. A method of incorporating a transition metal element in a glass body produced by the flame hydrolysis technique is described in U.S. Pat. No. 3,859,073 to SCHULTZ.
Also known in the art is the optical absorption of the transition metal elements in vitreous silica, as described by Peter C. SCHULTZ in xe2x80x9cOptical Absorption of the Transition Elements in Vitreous Silicaxe2x80x9d, J. Am. Ceram. Soc., Vol. 57, No. 7, pp. 309-313, 1974.
Knowledge of this has permitted the manufacture of high-attenuation fibers. These high-attenuation fibers can be tailored to produce a controlled level of attenuation with a high degree of wavelength insensitivity, by properly selecting the material used as a dopant, the dopant concentration and the length of the attenuation fiber.
A high-attenuation fiber is used to produce optical fiber attenuators which can be inserted in optical transmission systems to attenuate the amount of optical power present in a fiber link. The most common use for a high attenuation fiber is to either attenuate the detected optical signal level down to a receiver""s optimum detection sensitivity, or to act as a reflection-less terminator at the end of unused fibers. Examples of high attenuation fibers, and methods for making the same are described in U.S. Pat. No. 4,881,793 (Tarbox); U.S. Pat. No. 5,572,618 (DiGiovanni et al.); and U.S. Pat. No. 5,633,974 (Chia). Essentially, these high-attenuation fibers are made of a single-mode fiber, where the core is doped with, usually, a transition metal in order to increase the absorption in the core.
The high-attenuation fibers are usually a few centimeters long and are provided with connectors at each end to facilitate connection to a low-loss optical fiber. The fabrication of the high-attenuation fiber connectors implies the assembly of the high-attenuation fiber secured in a connector ferrule of precise diameter. The high-attenuation fiber is thus usually connected between the output end connector of a transmission fiber and the input end connector of a detection fiber.
The tolerance on standard fiber diameters is usually less than xc2x11 micron and the core-cladding concentricity is better than 1 micron. Also, the tolerance on the ferrule hole diameter is usually better than xc2x10.5 micron. Generally, a fiber with small diameter will be off-centered in the ferrule of bigger hole diameter. Thus, two connectors made with the same (or different) fibers, will usually exhibit offset-induced losses when they are connected together.
This phenomenon is known in the art and may cause modal interference. It has been found that when a short piece of single-mode fiber is connected between a transmission fiber and a detection fiber, most of the optical power of the fundamental mode of the transmission fiber is coupled into the fundamental mode of the short piece of fiber. However, because of the small discontinuity due to the misalignment of the joint, some of the power of the fundamental mode of the transmission fiber is coupled into higher-order modes of the short piece of fiber. The fundamental mode and the higher-order modes propagate along the short fiber piece with different propagation delays and reach the junction of the short fiber piece and the detection fiber out of phase. Again, most of the optical power of the fundamental mode of the short fiber piece is coupled into the fundamental mode of the detection fiber. Also, because of a misaligned joint, some of the optical power of the higher-order modes of the short piece of fiber is coupled into the fundamental mode of the detection fiber, where it interferes with the optical power coupled from the fundamental mode of the short piece of fiber. The out-of-phase component varies with wavelength and results in an interference effect that can be observed as an oscillation in the detected signal from the detection fiber measured as a function of wavelength. For a greater discussion on modal interference, see S. HECKMANN, xe2x80x9cModal Noise in Single-Mode Fibres Operated Slightly Above Cutoffxe2x80x9d, Elect. Let., Vol. 17, No. 14, pp. 499-500, 1981, and K. ABE, Y. LACROIX, L. BONNELL and Z. JAKUBCZYK, xe2x80x9cModal Interference in a Short Fiber Section: Fiber Length, Splice Loss, Cutoff and Wavelength Dependencesxe2x80x9d, J. Light. Technol., Vol. 10, No. 4, pp. 401-406, 1992.
Modal interference is practically unpredictable and is of great concern to the optical fiber systems designers. The common solution to the modal interference problem is to use at least a one meter long fiber piece after each discontinuity (connector or splice) to eliminate the optical power coupled into the higher-order modes. Other solutions have been proposed with a special fiber design to control modal noise in short fiber sections. One such design includes an outer cladding layer that has a high refractive index and a high attenuation (see for example U.S. Pat. No. 4,877,306 to KAR). The outer cladding effectively traps higher-order modes. Another proposed solution has been to use doubly clad optical fibers with a low-index inner cladding. Such a fiber is usually referred to as a W-type fiber. This type of fiber increases the transmission loss of the higher-order modes and encourages leakage of such higher-order modes in the outer cladding. For a detailed discussion on the propagation in doubly-clad single mode fibers, see M. MONERIE, xe2x80x9cPropagation in Doubly Clad Single-Mode Fibersxe2x80x9d, IEEE J. Quantum Electronics, Vol. QE-18, No. 4, pp. 535-542, 1982 and S. KAWAKAMI and S. NISHIDA, xe2x80x9cPerturbation Theory of a Doubly Clad Optical Fiber with a Low-index Inner Claddingxe2x80x9d, IEEE J. Quantum Electronics, Vol. QE-11, No. 4, pp. 130-138, 1975.
An example of the use of this design can be found in U.S. Pat. No. 5,241,613 (Li et al.), in order to reduce modal interference.
However, to Applicant""s knowledge, no fiber design has been proposed which attenuates the fundamental mode and reduces modal interference in the same device of relatively small length.
It is thus an object of the invention to provide an optical attenuating element which also reduces modal interference, and more specifically, an optical device which combines a controlled level of attenuation, a high degree of wavelength insensitivity and a substantial decrease in modal interference problems. In accordance with the invention, this object is achieved with an optical attenuating element for attenuating and eliminating modal interference comprising a core having a core diameter and a core refractive index; an inner cladding having an inner cladding outer diameter less than ten times the core diameter and a refractive index less than the core refractive index; and an outer cladding having a refractive index higher than the inner cladding refractive index; where at least one part of at least one of said core, said inner cladding and said outer cladding is doped with at least one absorbing element.